This invention relates generally to xerographic copying apparatus and, more particularly, to a contact fusing system for fixing electroscopic toner material to a support member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual image can be either fixed directly upon the photosensitive member or transferred from tile member to a sheet of plain paper with subsequent affixing of the image thereto.
In order to permanently affix or fuse electroscopic toner material onto a support member by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to be absorbed to some extent into the fibers of the support member which, in many instances, constitutes plain paper. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be firmly bonded to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll to thereby effect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner whereat the toner particles in the image areas of the toner would liquefy and cause a splitting action in the molten toner to thereby result in hot offset. Splitting occurs when the cohesive forces holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting", has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene, commonly known as Teflon, to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of several mils and the thickness of the oil being less than 1 micron. Silicone based oils, which possess a relatively low surface energy, have been found to be materials that are suitable for use in the heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to thereby form an interface between the roll surface and the toner images carried on the support material. Thus a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
The release agent material can be applied directly to the heated fuser roll or it can be applied indirectly via the pressure roll. Use of a donor roll RAM (Release Agent Management) system is one way of applying the release agent to the fuser roll. Donor roll RAM systems have been used as part of roll fuser apparatus for some time. Such a RAM system is disclosed in U.S. Pat. No. 4,214,549 issued on Jul. 29, 1980 to Moser This patent illustrates a heat and pressure roll fusing apparatus for fixing toner images to copy substrates, the toner comprising a thermoplastic resin. The apparatus includes an internally heated, fuser roll cooperating with a backup or pressure roll to form a nip through which the copy substrates pass with the images contacting the heated roll. The pressure roll is the softer of the two rolls, therefore, the nip is formed by the harder fuser roll indenting the softer pressure roll. The heated fuser roll is characterized by an outer layer or surface which by way of example is fabricated from a silicon rubber or Viton material to which a low viscosity polymeric release fluid is applied. Release fluid is contained in a sump from which it is dispensed by means of a metering roll and a donor roll, the former of which contacts the release fluid in the sump and the latter of which contacts the surface of the heated fuser roll.
U.S. Pat. No. 3,716,221 issued on Feb. 13, 1973 to Gorka et al discloses a heat and pressure fuser roll wherein the heated fuser roller includes a fusing roller having a resilient fusing blanket supported on the periphery thereof and heating means to heat the fusing blanket to a temperature sufficient to fuse the particulate material on a copy sheet. A backup roller is urged toward engagement with the deformable fusing blanket to press the receptor sheet carrying the particulate material into contact with the fusing roller. The fuser roller is coated with an offset preventing liquid which is applied thereto from the backup roller at predetermined intervals during operation of the device. The offset preventing liquid is applied to the backup roller via a wick, one end of which is immersed in a quantity of the liquid which is contained in a receptacle. The application of the liquid to the backup roller is controlled such that it is applied once every eleventh revolution of the fuser roller. In other words ten copy sheets are passed through the fuser and then the fuser and backup rollers are rotated an eleventh time without a copy sheet passing therebetween at which time the liquid is applied to the fuser roller via the wick and backup roller.
U.S. Pat. No. 3,929,094 granted to Raghulinga R. Thettu on Dec. 30, 1975 discloses a contact fuser assembly wherein a heater is provided in a sump of a RAM system for melting a release agent material which is solid at room temperature. The melted release agent is applied to a heated fuser roll supported for pressure contact with a Viton (Trademark of E. I. dupont de Nemours and Company) coated pressure roll.
U.S. Pat. No. 3,934,547 granted to Jelfo, et al. on Jan. 27, 1976 discloses a contact fuser assembly for use in an electrostatic reproducing apparatus including an internally heated fuser roll structure comprising a rigid, thermally conductive core which is coated during operation of the assembly with a thin layer of a normally solid thermally stable material with subsequent application of a liquid release agent to the coated core. In the preferred embodiment of the invention the coating material comprises a fluorocarbon telomer such as Vydax 1000 and the liquid release agent comprises a liquid silicone oil.
U.S. Pat. No. 3,941,085 granted to Hattier, et al on Mar. 2, 1976 discloses an apparatus in which release material is applied to a heated fuser member. A backup member is in communication with the fuser member and a sheet of support material having particles thereon passes therebetween. The particles on the sheet of support material contact the fuser member. The apparatus reciprocates a bar of release material into and out of contact with the fuser member.
U.S. Pat. No. 4,770,116 granted to Rabin Moser on Sept. 13, 1988 discloses a heat and pressure roll fusing apparatus for fixing toner images to copy substrates, the toner comprising a thermoplastic resin. The apparatus includes an internally heated fuser roll cooperating with a bare metal backup or pressure roll to form a nip through which the copy substrates pass with the images contacting the heated roll. The heated fuser roll is characterized by an outer layer or surface which by way of example is fabricated from a silicon rubber or Viton material to which a low viscosity polymeric release fluid is applied. Release fluid is contained in a sump and the pressure roll is partially immersed in the fluid. Thus, the release fluid is applied to the surface of the internally heated fuser roll via the bare metal pressure.
RAM systems such as the ones discussed above are either spill proof and expensive or vice versa. Moreover, the use of liquid release agents such as silicone oil with silicone rubber coated fuser or pressure rolls results in roll swell. As will be appreciated, it is highly desirable that a RAM system be spill proof, highly reliable and be capable of dispensing a low rate of oil. Also, is desirable that the release agent not cause either the pressure or fuser roll to swell.